Learn how to calculate the day of the week for any date you can think of with this impressive mental trick and some practice. In this It’s Okay to Be Smart video, Universal Calendar Puzzle, Joe Hanson demonstrates the Doomsday rule, an algorithm that makes this computation possible without the help of a calendar, a computer, or a calculator. Via Wikipedia:

The Doomsday algorithm for mental calculation was devised by John Conway in 1973 after drawing inspiration from Lewis Carroll‘s perpetual calendar algorithm. It takes advantage of each year having a certain day of the week, called the doomsday, upon which certain easy-to-remember dates fall; for example, 4/4, 6/6, 8/8, 10/10, 12/12, and the last day of February all occur on the same day of the week in any year. Applying the Doomsday algorithm involves three steps:

  1. Determination of the anchor day for the century.
  2. Calculation of the doomsday for the year from the anchor day.
  3. Selection of the closest date out of those that always fall on the doomsday, e.g., 4/4 and 6/6, and count of the number of days (modulo 7) between that date and the date in question to arrive at the day of the week.

This technique applies to both the Gregorian calendar A.D. and the Julian calendar, although their doomsdays are usually different days of the week.

Also: Noneday, Oneday, Twosday, Threesday, Foursday, Fivesday, Sixturday. Download It’s Okay to Be Smart’s Universal Calendar Puzzle Cheat Sheet .pdf, watch the video, and start practicing.

Follow this video with more math tricks: How to multiply numbers by drawing lines and how high can you count on your fingers?

Bonus algorithm: What’s the fastest way to alphabetize your bookshelf?

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